A healthy individual's heart rate (HR) increases intrinsically as the individual's physical activity or emotional level increases. This intrinsic chronotropic response to increased activity or work load allows the individual's heart to pump oxygen-rich blood more quickly throughout the body. As used herein, the phrase “chronotropic response” means a change in the rate of heart contraction. The phrase “intrinsic heart rate” means the inherent rate of heart contraction without artificial pacing. For example, the intrinsic heart rate may be the rate set by the sinoatrial or atrioventricular nodes.
Activity and work load are used interchangeably herein to represent an individual's current state of exertion, ranging from a resting state to maximal exertion. An individual's heart rate will increase with increased activity or work load until the heart rate reaches a maximum heart rate. This maximum heart rate can be approximated using the following Equation 1 to calculate a maximal predicted heart rate (PHRmax).PHRmax=220−age  (1)As shown by Equation 1, the PHRmax decreases with age.
The difference between an individual's PHRmax and resting heart rate (HRresting), or the individual's heart rate during relaxation, is termed the individual's heart rate reserve (HRR) and can be calculated as shown in Equation 2.HRR=PHRmax−HRresting  (2)The individual's HRR is a measure of the individual's capacity to increase heart rate and can be expressed in terms of % HRR, calculated for any given level of activity HRstage as shown in Equation 3.
                              %          ⁢                                          ⁢          HRR                =                                                            HR                stage                            -                              HR                rest                                                                    PHR                max                            -                              HR                rest                                              ×          100          ⁢          %                                    (        3        )            
Another relevant measurement of an individual's activity level is the metabolic equivalent (MET), which measures the oxygen uptake of the individual. A MET is equal on average to 3.5 mL of O2/kg per minute. By measuring an individual's oxygen consumption and converting the oxygen consumption into METs, the individual's exercise capacity can be measured. An individual's metabolic reserve (MR) is a measure of the individual's total metabolic capacity. An individual's MR can be calculated by taking the difference between an individual's maximal and minimal MET values, as shown in Equation 4.MR=METmaximal−METresting  (4)A percentage MR for any given stage of metabolic activity METSstage can be calculated as shown in Equation 5.
                              %          ⁢                                          ⁢          MR                =                                                            METS                stage                            -                              METS                rest                                                                                      METS                  maxima                                ⁢                1                            -                              METS                rest                                              ×          100          ⁢          %                                    (        5        )            
A linear relationship between % HRR and % MR has been identified and is illustrated in FIG. 1. In FIG. 1, the % HRR is plotted against % MR for a healthy individual. As illustrated, a healthy individual exhibits a linear rise in the percentage of HRR equal to that of the percentage of MR. In other words, as an individual's percentage of metabolic reserve utilized increases, the individual's percentage of heart rate reserve utilized increases proportionally. This relationship is mathematically represented for any given activity level by Equation 6 and is shown in simplified form in Equation 7.
                              HR          stage                =                                                            (                                  220                  -                  age                  -                                      HR                    rest                                                  )                            ×                              (                                                      METS                    stage                                    -                  1                                )                                                                    METS                maximal                            -              1                                +                      HR            rest                                              (        6        )                                          HR          stage                =                              (                          HRR              ×              %              ⁢                                                          ⁢              MR                        )                    +                      HR            rest                                              (        7        )            
The relationship between % HRR and % MR, as illustrated in FIG. 1 and Equations 6 and 7, is termed the metabolic-chronotropic relation (MCR). The MCR generally illustrates that as a healthy individual's physical activity or work load increases, the individual's intrinsic heart rate and associated cardiac output increases proportionally.
However, in some individuals with heart abnormalities, such as those suffering from sick sinus syndrome, an increase in activity or work load does not always evoke an associated proportional increase in heart rate. Studies on individuals suffering from sick sinus syndrome have shown that, although some individuals exhibit normal intrinsic chronotropic responses to increased activity at certain times, these individuals are intermittently chronotropic incompetent, resulting in situations in which the individual's heart rate increases little or not at all for an increase in work load.
A cardiac rhythm management (CRM) system is a common solution for problems associated with a heart's inherent electrical functions. Therefore, for individuals suffering from heart abnormalities such as sick sinus syndrome, a CRM system including adaptive-rate pacing may be used to artificially increase the individual's heart rate during increased activity and chronotropic incompetent periods, thereby providing increased hemodynamic benefits for these individuals.
The fundamental components of a CRM system include a CRM device, such as a cardiac pacing device or a cardiac resynchronization device, which includes a pulse generator for creating electrical impulses to stimulate the heart. Also included are one or more electrodes for delivering the electrical impulses and sensing the heart's intrinsic electrical activity.
A CRM system may further provide adaptive-rate pacing by including an adaptive-rate pacing device with one or more adaptive-rate sensors. Adaptive-rate pacing utilizes the one or more adaptive-rate sensors to sense an increase in activity and artificially increase heart rate during the increased activity. Adaptive-rate pacing is premised on the metabolic-chronotropic relation, as illustrated in FIG. 1 and described above, in that adaptive-rate pacing assumes that an increase in activity is proportional to an increase in heart rate. Therefore, during adaptive-rate pacing, the one or more adaptive-rate sensors are used to measure an individual's increased activity or work load, and based on the measurements, an appropriate heart rate for the given activity is approximated.
Presently, there are three major types of commercial adaptive-rate sensors available, including activity sensors, minute ventilation sensors, and QT interval sensors. All three types of sensors use different physiological criteria to measure changes in activity or work load and therefore an increased need for cardiac output. When an adaptive-rate pacing device determines that an individual has increased his or her work load, the adaptive-rate pacing device increases the frequency at which electrical impulses are communicated to the individual's heart, thereby increasing the individual's heart rate and cardiac output.
Illustrated in FIG. 2 is a second graph whereon a sensor-indicated rate (SIR) is plotted versus a sensor response. The SIR is the rate at which an adaptive-rate pacing device causes electrical impulses to be communicated to the heart and can range from zero (i.e., no pacing is provided) to a maximum sensor rate (MSR), which, for example, may be set at an individual's maximal predicted heart rate (see Equation 1). A sensor response is a signal communicated by the adaptive-rate sensor to the adaptive-rate pacing device, the signal being proportional to an increase in an individual's activity or work load.
The line provided in FIG. 2 illustrates the linear relationship between the sensor response and the SIR. For example, for a sensor response of a measured work load WLA, the adaptive-rate pacing device will provide a SIR of SIRA. A SIR for any given sensor response can be calculated using Equation 8.Sensor-Indicated Rate=Sensor Response×Response Factor  (8)The response factor (RF) is a constant used to relate the sensor response of the adaptive-rate sensor to the SIR provided by the adaptive-rate pacing device. An increase in the RF will increase the SIR for a given sensor response. Conversely, a decrease in the RF will decrease the SIR for a given sensor response. This increase or decrease in the sensor-indicated rate translates into an increase or decrease in the pacing of the heart during chronotropic incompetence for a given level of activity.
Therefore, an adaptive-rate sensor can approximate a heart rate for a given level of activity by measuring the work load, provide a sensor response proportionate to the measured work load, and correlate the sensor response to a SIR by multiplying the sensor response by the response factor, as shown by Equation 8. The response factor determines how an adaptive-rate sensor responds to a given work load and therefore dictates the slope of the line shown in FIG. 2.
In FIG. 3, a graph similar to that shown in FIG. 2 illustrates two responses for two adaptive-rate pacing devices A and B that have been calibrated differently. As shown, a response factor for the adaptive-rate pacing device A is greater than a response factor for the adaptive-rate pacing device B. Because the response factor for pacing device A is greater than the response factor for pacing device B, the slope of line A is steeper than the slope of line B. Therefore, for a given work load and associated sensor response, such as WL1, the sensor-indicated rate, e.g., SIR2, for the adaptive-rate sensor A is greater than the sensor-indicated rate, e.g., SIR1, for the adaptive-rate sensor B. The two sensor-indicated rates SIR1 and SIR2 differ because the response factors for the two adaptive-rate pacing devices A and B are different. In addition, a maximum sensor rate MSR2 for the adaptive-rate pacing device A is configured to be greater than a maximum sensor rate MSR1 for the adaptive-rate pacing device B.
Adaptive-rate pacing must be customized for each individual. An adaptive-rate pacing device is typically calibrated by a caregiver. Such variables as maximum sensor rate, response factor, and sensor rate target (described below) are typically configured using physiological data collected from the individual as well as one or more equations of general applicability. For example, a caregiver may use Equation 1 to set the MSR and Equation 6 to approximate an individual's heart rate at a given work load in an attempt to define an appropriate RF.
The practitioner may also attempt to optimize calibration by setting a sensor rate target (SRT). The SRT is an approximation of the individual's daily achieved maximum heart rate. The SRT is typically set using subjective data provided by the individual. For example, the SRT may be set using the individual's desired target heart rate during exercise. The SRT is used in conjunction with an average daily maximum paced heart rate ( MPHR) calculated by the CRM system. The MPHR is calculated by storing a maximum paced heart rate for each day for an individual over a specific time period. The daily maximum paced heart rates are then averaged to calculate the MPHR. Then, based on the SRT and MPHR, the adaptive-rate pacing device is calibrated using Equations 9 and 10 below.If MPHR>SRT, then⇓RF  (9)If MPHR<SRT, then⇑RF  (10)As illustrated by Equation 9, if the MPHR is greater than the SRT, the RF is decreased to better approximate daily activity. Conversely, if the MPHR is less than the SRT, then the RF is increased.
Calibration of a CRM system, and particularly adaptive-rate pacing, as illustrated above, involves clinical experimentation as well as complex calculations. The calibration can take valuable time to perform and may encourage practitioners to take short-cuts in the calibration process. In addition, the complexity of the calibrations can cause mistakes to be made. As illustrated by the responses A and B shown in FIG. 3, the calibration of an adaptive-rate pacing device can have a significant effect on how an adaptive-rate pacing device responds to a given activity or work load. This can lead to episodes of hemodynamic deficiency, wherein an individual may become fatigued while performing at a certain work load because the individual's heart rate does not represent the typical chronotropic response for the given work load. Further, calibration of the CRM system may not adequately account for progressive changes in an individual's needs subsequent to calibration.
Therefore, calibration of an adaptive-rate pacing device remains a challenging task. The methods are complex, subjective, and do not automatically adapt to account for changes in an individual's cardiac output needs. In addition, there is the possibility that calibration will be improperly performed.